Optimum Operating Conditions By Co-Electrolysis in Solid Oxide Cell for Fischer-Tropsch Synthesis

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Highly efficient liquid fuel production can be expected by integrating the Fischer-Tropsch (FT) synthesis process at the latter stage of the solid oxide electrolysis cell (SOEC) process and integrating the heat transfer between the processes. However, it is difficult to achieve the high efficiency and durability conditions in SOEC process with the high-yield conditions in FT synthesis process. In this study, the SOEC operating conditions were investigated to improve the efficiency and durability of the integrated production process by thermodynamic equilibrium calculations of the gas composition at the SOEC outlet (FT synthesis inlet) and experiments using a single cell.Gas compositions of H2/CO=2.0 and high H2 and CO production are favorable for FT synthesis. To achieve these conditions at the SOEC outlet, the SOEC inlet gas must be close to (H2+H2O)/CO2=2.0, and reactant utilization must be high. However, these conditions are likely to cause carbon deposition in SOEC, which can lead to cell damage. An effective way to prevent carbon deposition is to increase the cell temperature because carbon deposition is an exothermic reaction. High temperature conditions have the advantage of lowering the cell voltage in co-electrolysis and preventing carbon deposition, but the life of the cell components may be shortened. Promoting methanation was also an effective way to prevent carbon deposition because of consuming CO, but it may cause lower yields of liquid fuel from FT synthesis. Under pressurized conditions, the volume reduction reaction which includes methanation and carbon deposition is accelerated Therefore, the risk of carbon deposition under pressurized conditions has become highly dependent on the SOEC operating conditions which determine whether methanation or carbon deposition is more promoted. These results clarify the risks associated with changing the operating conditions by co-electrolysis in SOEC and can be used to guide the optimum operating conditions.